Hospitalized bedridden patients are at a high risk of aspirating and choking on secretions, food, medicines or gastric contents. These risks can be lowered by closely monitoring the absolute angular orientation of the patient's upper body. For example, patients on ventilators are especially vulnerable to developing severe pneumonia after aspirating gastric contents. Numerous studies have shown that such cases of pneumonia, known as ventilator-associated pneumonia, often can be prevented by elevating the plane of the upper body of the patient by approximately 30 degrees above the local horizontal plane. The plane of the upper body is defined as a plane that that passes through the line between the two shoulder joints and runs parallel with the spine. The local horizontal plane is defined as a plane normal to the local gravity vector. For the purposes of this disclosure it is assumed that the plane of the head of a hospital bed is parallel with the plane of the upper body and these terms may be used interchangeably.
While elevating the plane of the head of the bed is simple in principle, it is elusive in practice. This is because there are many reasons to lower the patient's upper body plane during the course of patient care. Some of these reasons include transport, bathing, or bedside medical procedures. Frequently, after the patient's upper body plane has been lowered for some reason and the caregiver forgets to elevate the head of the patient back in the elevated position. As a result, inadequate head elevation for at-risk patients is relatively common over the course of the treatment period. The disclosed subject matter helps to avoid this problem.
Known art, such as U.S. Pat. No. 7,117,607 by Horgan, discloses a device utilizing an “electronic inclinometer” to measure the head of bed angle to alert caregivers of dangerous head of bed angles for patients. However, known methods use a single axis inclinometer as the angle sensing device. These devices may suffer from the need for precise placement of the single sensing axis parallel with the head of bed axis of rotation.
Known art, such as U.S. Pat. No. 7,340,955 by Manninen, discloses a capacitive acceleration sensor arrangement that takes advantage of digital integration of the acceleration measurements. As quoted in Manninen:                “An advantage of the acceleration sensor measuring circuitry according to the present solution is the replacement of an analog integrator with a digital integrator, whereby the function of the circuit is unaffected by offset errors in the analog integrator. The integrators are also less sensitive to interference, in particular in the multiplexed application. The integrators can be implemented by means of modern sub-micron CMOS technology. The circuitry can also be designed to work at extremely low voltages, below 2.0 volts.        Sensitivity variations in connection with the processing of the sensor element can be calibrated away by adjusting the transfer function of the D/A converter. The digital output signal of the circuit is directly the output of the integrator, which can be transferred for further processing as a signal in either parallel or serial form. The power consumption of the D/A converter can achieve a very low level by means of CMOS technology (CMOS, Complementary Metal Oxide Semiconductor) by using the SC circuit technique (SC, Switched Capacitor).        A further advantage of the circuitry is the replacement of a charge amplifier by a fast comparator. The problem with a charge amplifier is the need for a large uniform range of operation because of the variation in sensor capacitances, as well as, in the multiplexing environment, a need for a wide bandwidth, which makes designing the charge amplifier a challenge and tends to increase the power consumption of the prestage. These problems largely disappear by means of a fast comparator.”        